1. Field of the Invention
The present invention relates to measuring differential group delay in an optical fiber connection.
The invention finds applications particularly in the fields of telecommunications and optical fiber metrology. In particular, it is used for qualifying an optical fiber connection for the recipe of high rate and long-range wavelength division multiplexing (WDM) optical transmission systems with binary phase shift key (BPSK) or differential phase shift key (DPSK) modulation.
2. Description of the Prior Art
In an optical fiber transmission system, the whole connection, including line fiber sections and chromatic dispersion compensation fiber sections, as well as various elements of the system, such as multiplexers, demultiplexers, amplifiers, and OADM (Optical Add Drop Multiplexer) canal insertion and extraction components, behaves like a birefringent medium that induces effects that are harmful for propagation of a polarized optical signal modulated by a digital signal and injected into the connection.
According to a simple model of propagation for a pulse I of the electrical field of a linearly polarized optical signal in an optical connection LO, shown on FIGS. 1A and 1B, the pulse I is broken down into a fast pulse IR and a slow pulse IL along fast AR and slow AL orthogonal polarization axes of the birefringent medium. The polarization state of the pulse I is associated with the inclination level of the pulse with respect to one of the polarization axes, more particularly the fast axis AR. For instance, for an inclination of 45°, the impulsion I power is equidistributed on both axes.
For ideal faultless propagation as shown on FIG. 1A, pulses IR and IL respectively associated with axes AR and AL at the emergence end of the optical connection LO are synchronous. However, as shown on FIG. 1B, as the optical connection is not perfect, the birefringence phenomenon results in an enlarged pulse IE being received at the emergence end of the connection resulting from the sum of pulses IR and IL respectively transmitted along the axes AR and AL. On one hand, the enlarged pulse IE depends on a variation of the different propagation times for the pulses IR and IL that is due to birefringence variations accumulated along the connection LO that induces a propagation delay T1 for the slow pulse IL compared to the fast pulse IR at the emergence end of the connection LO. On the other hand, the enlarged pulse IE depends on the distribution of the pulse I power between the polarization axes AR and AL, on the variation of the position of the polarization axes along the connection, on the optical signal wavelength of the pulse I and on the environment conditions for the connection LO. The delay T1, called a differential group delay DGD, must be kept within a tolerance range specified by a maximum delay DGDmax that is determined as a function of the data rate, the coding, and the modulation format of the signal injected into the connection. This maximum delay must be less than the bit period 1/D, i.e., inversely proportional to the rate D in bit/s of the digital signal modulating the optical signal transmitted over the connection.
The differential group delay DGD is simultaneously variable over time and is a function of the optical signal wavelength. The differential group delay is an instantaneous magnitude which depends on numerous physical factors that can vary over time, such as temperature, and stresses locally applied to an optical fiber, etc. Knowing the instantaneous value of the differential group delay T1 of a connection is essential for determining the formula of WDM systems or to improve the quality of transmission.
The total dispersion of the modulated optical signal emerging from the transmission system through an optical fiber due to the polarization of the injected signal and to the birefringence of the system optical medium is characterized by a magnitude called polarization mode dispersion PMD. This magnitude corresponds to the average of the differential group delays DGD for all the polarization states and all the wavelengths of the signals transmitted through the connection over the duration of the polarization mode dispersion measurement.
Patent application WO 2007/074277 owned by applicants assignee, corresponding to patent application US 2009/0066937 A1, discloses an instantaneous measurement of the differential group delay T1 in an optical fiber connection with a direct modulation by pulses NRZ-OOK (Non-Return to Zero/On-Off Keying). The differential group delay is measured in a test channel of the connection corresponding to a specific wavelength of the optical signal without causing traffic rupture in the optical signals multiplexed to the other wavelengths in the connection channels.
A measuring system for measuring the differential group delay according to the above mentioned patent application comprises, at the entrance end to the connection, a transmitter for modulating an optical signal through the pulses of a binary test sequence having a given rate D in a test channel, and a first polarization controller suitable for scanning polarization states applied to the modulated signal. At the emergence end from the connection, the measuring system comprises a second polarization controller independent from the first controller, suitable for subjecting the modulated signal emerging from the connection to a scan through polarization states, a differential group delay emulator suitable for varying a variable additional differential group delay T2, and a digital oscilloscope suitable for determining the value T1+T2=1/D of the resulting modulated signal transmitted by the emulator. In the emulator, the polarized optical signal outgoing from the second polarization controller is separated by birefringence into a modulated optical signal delayed by T2 following the slow axis and a non-delayed modulated optical signal following the fast axis. At the output of the emulator, the delayed modulated optical signal and the non delayed modulated optical signal are combined in a resulting modulated signal applied to the digital oscilloscope. As the optical signal of the test channel directly undergoes modulation by the test sequence pulses, the resulting signal outgoing from the emulator has three levels. The maximum level in the resulting signal corresponds to the addition of two synchronous pulses of the signal delayed by T2 and the non delayed signal. The intermediate level of the resulting signal corresponds to the pulse either in the delayed signal or in the non delayed signal. By suitably tuning the polarization states of the controllers and by adjusting the delay inside the emulator, the delay T2 can be derived when the opening of the eye diagram in the resulting signal is maximum in the digital oscilloscope.
When the optical fiber connection is BPSK or DPSK phase modulated, the modulated optical signal propagating along the connection has constant amplitude. Between two consecutive binary periods of the signal, the phase shift of the optical signal is 0 or π according to the information bit state to be transmitted. Because of the polarization mode dispersion in the connection, the slow and fast components of the resulting phase modulated signal outgoing from the emulator have at any time t the same phase 0 or π or two opposite phases 0 and π. Since such slow and fast components are orthogonal, the total power of the resulting signal observed on the oscilloscope is thus constant. In the absence of amplitude modulation (including by pulses) of the signal intensity, an eye diagram of the resulting signal with plural amplitude levels can not be directly produced at the output of the emulator by the digital oscilloscope.